The following publications are believed to represent the current state of the art:
U.S. Pat. Nos. 653,629; 1,636,190; 2,607,944; 3,202,447; 3,203,447; 3,319,891; 3,332,045; 3,446,942; 3,427,675; 3,475,588; 3,632,042; 3,524,044; 3,537,900; 3,643,193; 3,711,679; 3,716,886; 3,747,500; 3,888,412; 3,977,436; 3,979,068; 4,090,668; 4,088,269; 4,106,508; 4,159,026; 4,212,425; 4,253,493; 4,295,111; 4,306,589; 4,403,765; 4,489,863; 4,508,957; 4,524,797; 4,534,539; 4,561,632; 4,574,841; 4,616,780; 4,638,525; 4,690,371; 4,815,662; 4,834,289; 4,877,186; 4,832,262; 4,922,570; 4,946,009; 4,999,550; 5,012,977; 5,034,714; 5,118,040; 5,134,266; 5,141,157; 5,141,160; 5,173,586; 5,203,049; 5,254,083; 5,271,120; 5,318,071; 5,345,968; 5,351,934; 5,354,965; 5,383,247; 5,423,486; 5,467,522; 5,509,606; 5,561,882; 5,636,407; 5,650,080; 5,673,360; 5,711,486; 5,711,487; 5,727,769; 5,762,278; 5,784,751; 5,823,439; 5,881,428; 5,903,953; 5,927,608; 5,944,910; 5,947,348; 5,957,384; 5,965,950; 5,979,796; 5,988,523; 5,988,529; 6,008,474; 6,024,803; 6,029,908; 6,032,324; 6,050,503; 6,082,632; 6,133,546; 6,164,564; 6,155,493; 6,199,587; 6,220,524; 6,223,951; 6,236,019; 6,257,500; 6,281,649; 6,286,174; 6,330,497; 6,781,056; 6,841,739; 6,896,199; 6,615,438; 6,669,109; 6,892,417; 7,108,754 and 7,171,716.
Published PCT Applications: WO 2007/046106, WO 2005/076735, WO 2004/035358, WO 02/092237, WO 00/27540 and WO 98/58826.